Should the rollback feature be removed in favor of something more cloud-friendly?
No. Some people do want it (same with the prepare). The uses are pretty special though, when you bave complete control of all updates going to Solr and you know exactly when commits happen, etc.
In general, one should use something like deleteByQuery to remove a bunch of added updates (however that only works if the documents added are new and did not overwrite any older ones). Real transaction isolation is not a target feature for Solr.